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Abstract:

The invention relates to a device for controlling regenerative burners
used as a firing device, particularly for iron and steel product heating
furnaces or for radiating tubes for continuous strip steel processing
lines, according to which the supply of at least one of the fluids
involved in the combustion (fuel and comburent) is carried out through a
rotary injector (12) rotating by means of a rotatable actuator (M) so as
to supply fluid alternately to one and then the other of the burners (B1,
B2), the rotary injector (12) being placed on the comburent inlet duct
(6), particularly air, and is provided to partially obstruct the supply
pipe (19.1; 19.2) to the burner (B1; B2) such that one portion of the
fumes from a regenerator (2.2; 2.1) for the non-operating burner (B2; B1)
is led to the operating burner (B1; B2) and such that the other portion
of the fumes is discharged to the stack.

Claims:

1. A device for controlling regenerative burners used as firing
equipment, particularly for furnaces to reheat steel making products or
radiant tubes for lines for the continuous treatment of steel strip,
wherein at least one of the fluids involved in combustion (the fuel and
the oxidant) is supplied using a rotary injector by means of a rotary
actuator so as to supply one of the burners then the other, in turn, the
inlet of air and the outlet of flue gases for a burner are performed by
one and the same supply piping, the rotary injector is positioned on the
inlet pipe for oxidant, particularly air, and is designed to partially
shut off the supply piping leading to the burner so that some of the flue
gases from a regenerator of the burner that is switched off are drawn
toward the burner that is switched on and the other proportion of the
flue gases is discharged to the exhaust.

2. The device as claimed in claim 1, wherein the recirculation of a
proportion of the combustion flue gases toward the burner that is
switched on is performed by an entrainment effect associated with the
speed of the fluid on the exhaust side of the injector.

3. The device as claimed in claim 1 or 2, wherein the injector comprises
a cylindrical hollow body which, at one end, has a frustoconical nozzle
which opens radially via a small-diameter outlet orifice, it being
possible for the nozzle to be positioned facing the inlet of an air pipe
leading to a burner, so that flue gas entrainment zones are created by
the ejection of air through the outlet orifice.

4. The device as claimed in claim 3, wherein the injector comprises:
toward its end furthest from the nozzle, a part positioned in a
cylindrical housing produced in the form of a cold air valve with two
diametrically opposed orifices provided in the part, and one orifice
provided in the wall of the housing level with the orifices.

5. The device as claimed in claim 4, wherein a ring made of a material
with a low coefficient of friction is positioned between the body of the
injector and the wall of the housing.

6. The device as claimed in claim 5, wherein the ring is made of bronze.

7. The device as claimed in claim 1, wherein the rotary actuator consists
of an electric motor.

Description:

[0001] The invention relates to a device for controlling regenerative
burners which are used as firing equipment, for example for reheating
furnaces, particularly reheating furnaces for reheating steelmaking
products, notably slabs, blooms, slugs or billets, to raise the product
that is to be reheated to the temperature required for rolling, and
radiant tubes for lines of continuous processing of steel strip, for
example annealing lines and galvanizing lines.

[0002] Regenerative burners are systems with high combustion efficiency
because of the fact that one of the two fluids involved in the
combustion, generally the air, is preheated to a high temperature through
the regenerative matrixes of the burners. The fact that they operate in
pairs as burner and exhaust alternately means that they require a special
valving system.

[0004] In the accompanying drawings, FIG. 1 shows a schematic vertical
cross section through a pair of regenerative burners B1, B2 mounted
facing one another on longitudinally opposed walls of a furnace.

[0005] Each burner comprises a fuel injection tube 1.1, 1.2 and a
regenerative matrix 2.1, 2.2 formed of a compact mass of
heat-accumulating materials. On the opposite side of each matrix to the
combustion zone there are provided, respectively, an inlet duct 3.1, 3.2
for oxidant, generally air, and an outlet duct 4.1, 4.2 for the flue
gases.

[0006] An electrically operated valve 5.1, 5.2 respectively is provided on
the air ducts 3.1, 3.2 which are supplied by a common inlet pipe 6. An
electrically operated valve 7.1, 7.2 is respectively provided on the
outlet ducts 4.1, 4.2 which meet up with a common flue pipe 8, connected
to an exhaust.

[0007] The supply of fuel is afforded via a common pipe 9, via branches
with respective electrically operated valves 10.1, 10.2 for the tubes 1.1
and 1.2.

[0008] When the burner B2 is firing, the fuel inlet valve 10.2, the air
inlet valve 5.2 and the flue gas discharge valve 7.1 are open. The other
valves are closed. The burner B1 is switched off and acts as the exhaust
and the combustion gases pass through the matrix 2.1 to be discharged via
the pipe 4.1, the valve 7.1 and the pipe 8.

[0009] When the burner B1 is firing, the burner B2 is switched off and
acts as an exhaust. The position of the electrically operated valves is
reversed and the combustion air passes through the regenerative matrix
2.1, where it is heated up, before being mixed with the fuel in order to
be burnt.

[0010] Thus, where just one air electrically operated valve is used on a
conventional burner, four electrically operated valves 5.1, 5.2, 7.1, 7.2
are needed on a regenerative pair, two of which are hot 7.1, 7.2.

[0011] Leaving aside the additional expense created by this system in
terms of cost of procurement, the cost of maintaining the valving system
is also high because the valves are very high duty because of the high
number of opening and closing cycles which typically occur every 30
seconds.

[0012] The hot valves which discharge the flue gases in particular present
a technical problem when they start to leak after a high number of
opening and closing cycles, something which ultimately occurs with these
hot valves. The lack of gastightness of the valve on the flue gas side
causes some of the combustion air to leak to the exhaust. This results in
a lack of air supplied to the burner. The soot produced as a result of
incomplete combustion fouls the regenerators, further limiting the
combustion air flow rate. The burners ultimately become completely
clogged.

[0013] Furthermore, the flue gases laden with unburnt matter meet up with
the leaked air in the pipework downstream and this leads to a risk of
postcombustion in the pipes.

[0014] Maintenance on these valves in the hot state soon becomes a
restricting factor which often dictates carrying out the combustion with
a considerable amount of excess air. However, these burners already have
a tendency to produce a great deal of nitrogen oxide. Poor control of the
excess air exacerbates this phenomenon.

[0015] On furnaces of the reheating type with multiple high-capacity
burners, the onset of this problem is sufficiently slow that action can
be taken before failure occurs.

[0016] On radiant tube systems the problem soon becomes unmanageable,
because each unit is likely to lose its control settings with no warning.
Finally, the use of low powers does nothing to simplify flow regulation.

[0017] The supply of fuel to the burners requires two electrically
operated valves 10.1, 10.2 which are likewise subjected to a very heavy
duty cycle with a large number of cycles of opening and of closing.

[0018] It is a key object of the invention to reduce if not eliminate the
operating defects mentioned hereinabove in spite of the high frequency of
opening and closing of the circuits.

[0019] U.S. Pat. No. 3,170,680 discloses a glassmaking furnace comprising
regenerators. Each regenerator supplies several fuel injectors. In this
type of furnace there are not, strictly speaking, any burners but there
are fuel injectors it being understood that the temperature level is such
that the fuel self-ignites. That document does not describe any air
injector but describes supply piping or ducting. The field of application
is different from that of the invention which relates to regenerative
burners, each being equipped with an individual regenerative matrix and
with an injector.

[0020] The proposed invention, which is particularly well suited to small
power units, makes it possible to avoid the abovementioned problems by
making it possible: [0021] to eliminate valves, particularly hot
valves, thereby avoiding any deterioration of such valves over time,
leading to a reduction in the costs of the installation, [0022] to better
manage the excess air, [0023] to reduce the nitrogen oxides produced
during combustion by using a flue gas recirculation device.

[0024] According to the invention, a device for controlling regenerative
burners used as firing equipment, particularly for furnaces to reheat
steelmaking products or radiant tubes for lines for the continuous
treatment of steel strip, is characterized in that: [0025] at least one
of the fluids involved in combustion (the fuel and the oxidant) is
supplied using a rotary injector by means of a rotary actuator so as to
supply one of the burners then the other, in turn, [0026] the inlet of
air and the outlet of flue gases for a burner are performed by one and
the same supply piping, [0027] the rotary injector is positioned on the
inlet pipe for oxidant, particularly air, and is designed to partially
shut off the supply piping leading to the burner so that some of the flue
gases from a regenerator of the burner that is switched off is drawn
toward the burner that is switched on and the other proportion of the
flue gases is discharged to the exhaust.

[0028] The regenerative burners are combined in pairs and operate in turn.
One and the same supply piping for a burner provides the supply of air
when a burner is switched on and takes away the flue gases from the other
burner of the pair when this first burner is switched off and the other
burner is switched on, and vice versa for this other burner of the pair.

[0029] Advantageously, the recirculation of a proportion of the combustion
flue gases toward the burner that is switched on is performed by an
entrainment effect associated with the speed of the fluid on the exhaust
side of the injector.

[0030] The injector may comprise a cylindrical hollow body which, at one
end, has a frustoconical nozzle which opens radially via a small-diameter
outlet orifice, it being possible for the nozzle to be positioned facing
the inlet of an air pipe leading to a burner, so that flue gas
entrainment zones are created by the ejection of air through the outlet
orifice.

[0031] The injector may comprise, toward its end furthest from the nozzle,
a part positioned in a cylindrical housing produced in the form of a cold
air valve with two diametrically opposed orifices provided in the part,
and one orifice provided in the wall of the housing, level with the
orifices.

[0032] In general, a ring made of a material with a low coefficient of
friction is positioned between the body of the injector and the wall of
the housing. The ring may be made of bronze.

[0033] The rotary actuator may consist of an electric motor.

[0034] Apart from the provisions set out hereinabove, the invention
consists of a certain number of other provisions that will be discussed
more fully hereinbelow with reference to an exemplary embodiment
described with reference to the accompanying drawings, but which is not
in any way limited. In these drawings:

[0035] FIG. 1 is a schematic vertical section through a pair of
regenerative burners according to the prior art.

[0036]FIG. 2 is a schematic vertical section through a pair of
regenerative burners with a control device according to the invention.

[0037] FIG. 3 is a vertical axial section, on a large scale, through the
rotary injector in the closed position, and

[0038]FIG. 4 is a vertical section, on a larger scale, through the nozzle
of the injector and a pipe inlet.

[0039] The invention is described with reference to FIGS. 2 to 4 of the
accompanying drawings. The description is given for an application to
radiant U-tubes 11, only the ends of the parallel legs of which have been
depicted. The description can be carried across in its entirety to other
applications of regenerative burners, such as for pre-rolling reheating
furnaces.

[0040] Those elements in FIG. 2 that are similar to elements described
with reference to FIG. 1 are denoted by the same numerical references and
not described again.

[0041] In the depiction of FIG. 2, the burner B1 is firing while the
burner B2 is switched off and acts as an exhaust.

[0042] The electrically operated valves that supply air and discharge flue
gases have been omitted.

[0043] According to the invention, the supply of combustion air is
achieved by means of a rotary injector 12 that turns by means of a
three-position 180° rotary actuator M which switches the air inlet
to the top burner B1 then switches combustion over to the bottom burner
B2. The third position is an intermediate position and corresponds to
both burners being switched off. The actuator M, generally consisting of
an electric motor, may be fitted with end-of-travel contacts so that its
position can be monitored.

[0044] The rotary injector 12 comprises a cylindrical hollow body 13
arranged coaxially inside a cylindrical housing 14 which supports it and
guides its rotation. Advantageously, a ring 15 (FIG. 3) made of a
material with a low coefficient of friction, notably bronze, is
positioned between the body 13 and the housing 14. In the example
illustrated in FIG. 2, the axis of the body 13 is horizontal, but a
different orientation, notably a vertical one, is possible.

[0045] The body 13 is closed at its axial end facing towards the actuator
M. This end is connected by a shaft 16 to the actuator. At its other end,
the body 13 comprises an injection nozzle 17 which opens radially into a
space 18.

[0046] The inlet of air 3.1 into and the outlet 4.1 of flue gases from the
burner B1 are performed by one single same pipe 19.1, also known as the
supply piping. The same is true of the burner B2 and the pipe 19.2. More
specifically, when the burner B1 is switched on, the pipe 19.1 supplies
air to the burner B1, whereas the flue gases from the burner B1 are
discharged by the pipe 19.2 of the burner B2 switched off. When the
burner B1 is switched off while the burner B2 is switched on, the pipe
19.1 discharges the flue gases from the burner B2, and the pipe 19.2
supplies air to the burner B2.

[0047] The axes of the pipes 19.1, 19.2 are aligned and are orthogonal to
the axis of rotation of the body 13. The ducts 19.1, 19.2 open into the
space 18. The outlet of the nozzle 17 may be positioned either facing the
duct 19.1 or, by rotation through 180°, facing the duct 19.2, or
in an intermediate switched-off position.

[0048] The part 13a of the body 13 that is positioned in the housing 14 is
produced in the form of a "cold air valve" V and in its cylindrical wall
has two diametrically opposed orifices 20a, 20b. The air inlet pipe 6
opens into the cylindrical wall of the housing 14 via an orifice 21
situated, in the direction of the axis of the body 13, level with the
orifices 20a, 20b. If a bronze ring 15 is provided (FIG. 3), it has an
orifice 15a positioned in line with the orifice 21.

[0049] The flue pipe 8 opens into the space 18 and there is enough of a
passage 22, on the opposite side to the nozzle 17, between the body 13
and the inlet of the pipe 8 for the flue gases to pass, in the direction
of the arrow F, toward the exhaust which has not been depicted.

[0050] The nozzle 17 has a frustoconical external wall the diameter of
which decreases as far as the outlet orifice 17a. As is clearly visible
in FIG. 4, when the nozzle 17 is positioned facing the inlet of a pipe
19.1 or 19.2, a configuration akin to that of a venturi is obtained, and
flue gas entrainment zones are created around the frustoconical outer
wall, through the ejection of air through the orifice 17a.

[0051] The cold air valve V situated at the end of the rotary injector is
of the 180° cylindrical type. Its purpose is to cut off the inlet
of air in the intermediate position and to supply air in the two
operating positions corresponding to the two burners. It remains of
simple construction insofar as it does not necessarily need to be
rigorously fluidtight when closed.

[0052] The injector 12, via its frustoconical nozzle 17 with small outlet
orifice 17a, allows the cold air to be accelerated. Combustion air is
generally supplied at a high pressure, of the order of 1000 daPa (namely
0.1 bar), thus preventing the air from passing directly into the flue
gases.

[0053] The speed of the air locally entrains a proportion of the flue
gases toward the burner, the other proportion being discharged to the
outside of the system. The flue gases are at a moderate temperature,
typically of around 200° C., because they have previously been
cooled as they passed through the regenerative matrix 2.2 of the burner
switched to the exhaust position.

[0054] The rotary injector 12 equipped with its cold air valve V is
particularly well suited to on/off operation of the burners. In this
case, there is no need to provide an additional air flow rate control
member upstream of the cold air valve V. For proportional operation of
the burners, the variation in air flow rate is achieved upstream of the
cold air valve of the injector, for example using a flow regulating
valve.

[0055] The way in which the device for controlling the regenerative
burners 1.1, 1.2 works with the rotary injector 12 is as follows:

[0056] In the position illustrated in FIG. 2, the burner B1 is switched on
and the burner B2 is switched off. The injector 12 occupies the position
for which the orifice 20b of the body 13 communicates with the cold air
inlet pipe 6 while the nozzle 17 faces the inlet of the pipe 19.1. The
cold combustion air arriving via the pipe 6 is directed via the nozzle 17
into the pipe 19.1. The narrowing of the cross section of the nozzle 17
causes the airspeed at the outlet 17a to increase and causes a proportion
of the hot flue gases arriving along the pipe 19.2 to be entrained so
that they can be discharged via the pipe 8.

[0057] The air/flue gases mixture obtained passes through the regenerative
matrix 2.1 which has accumulated heat during the earlier cycle.
Appreciably heated up as it leaves the matrix 2.1, it is used for
burning, under good conditions, the fuel which arrives through the tube
2.1. The hot flue gases flow through the radiant tube 11 and return,
having lost some of their heat, to the burner B2 which is switched off.
The still-hot flue gases pass through the matrix 2.2 where they give up
some of their heat, and are then directed by the pipe 19.2 to the outlet
pipe 8.

[0058] In the next cycle, the supply of fuel to the burner B1 is halted
while the supply of fuel to the tube 1.2 of the burner B2 is opened. The
injector 12 is turned through 180° so that the nozzle 17 opens
into the pipe 19.2. The operation is the reverse of that described
hereinabove.

[0059] The rotary injector 12 therefore acts like a distributor and serves
to separate air from flue gases by partially blocking off the supply
piping 19.1 (or 19.2) leading to the burner B1 (or B2) so that flue gases
are discharged to the exhaust having passed through the regenerator 2.2
(or 2.1) of the burner B2 (or B1) that is switched off, without being
entrained toward the burner B1 (or B2) that is in operation.

[0060] It is well known to those skilled in the art that recirculating
some of the flue gases in the oxidizing air has the effect of reducing
the nitrogen oxides formed at the time of combustion.

[0061] By blocking the supply piping 19.1 (or 19.2) leading to the burner
B1 (or B2) only partially, the speed that the injector imparts to the
combustion air allows some of the flue gases to be entrained,
contributing toward a reduction in nitrogen oxides.

[0062] Finally, as it is no longer beholden to the life of the
electrically operated valves, it becomes possible to reduce the duration
of the burner switchover cycle, allowing small-sized regenerators 2.1,
2.2 to be built.

[0063] A rotary injector similar to the one described previously in
respect of the combustion air can also be used for the fuel, as a
replacement for the electrically operated valves 10.1 and 10.2 in FIG. 1.
Such a rotary injector for the fuel would have no nozzle 17, but a single
orifice at the site of the nozzle. This then would offer double
recirculation of flue gases by the combustion air and by the fuel with a
view to reducing the level of NOx produced.